The present invention relates generally to systems and methods for simulating optical atmospheric turbulence effects in a laboratory, and more particularly to a liquid mercury optical scintillator providing real-time statistical and near wavelength independent control over the spatial intensity modulation of light sources used for laboratory analysis of optical receivers.
In the prior art, proper evaluation of optical receivers requires testing both in the laboratory and the field in order to evaluate the receivers under atmospheric conditions. The conditions spatially modulate lasers used as test sources in ways that affect receiver performance. Because a need exists to enhance receiver performance, statistical behavior of atmospheric scintillation has been widely studied, resulting in a substantial data base relevant to probability distributions and power spectra of spatial and temporal distribution patterns of laser and nonlaser sources. The statistics encompass a variety of sources, atmospheric conditions and regional locale. Analyses of optical receivers under these atmospheric statistical effects require tests under specific scintillation conditions, which is a problem in that statistical control over atmospheric turbulence is not possible.
In a system described by Parker et al (U.S. Pat. No. 4,930,352 titled "Reflective Membrane Optical Scintillator", the teachings of which are incorporated herein by reference), atmospheric turbulence effects are simulated by vibrational modes of an acoustically excited tightly stretched bounded membrane which spatially modulates the intensity of collimated light reflected off its surface. Acoustic energy is coupled into the membrane through an electromechanical transducer. Audio signals presented to the transducer set up nodal vibrational modes in the membrane, which result in angular distortion areas along the surface which redirect rays of incident light and change the spatial irradiance distribution of the overall light beam. The energy redistribution statistics depend on the applied acoustical spectrum and the shape and impedance of boundary conditions imposed on the membrane. With random frequency excitation, nonstationary irradiance fluctuations simulate naturally scintillated laser light. In Parker et at, lack of vibrational damping along the hard membrane boundaries result in excess coupling of energy into resonant vibrational modes which adversely modulate the collimated light beam reflected off the membrane surface and result in multimodal probability density functions that clearly lack a In-normal distribution envelope. Resonant modes can be significantly reduced and ln-normal statistical distributions can be achieved through tedious adjustments in acoustic driving frequencies, transducer location, membrane tension and mechanical damping along the membrane boundary; these adjustments are time-consuming and membrane-dependent. If a membrane breaks, the system must be readjusted before again producing In normal statistical distributions.
The invention solves or substantially reduces in critical importance problems with prior art systems as just suggested by providing a liquid mercury optical scintillation system for simulating turbulence effects of optical transmission through the atmospere. In accordance with a governing principle of the invention, spatial irradiance variations in the beam reflected from a vibrating liquid mercury pool exhibit probability distributions matching atmospheric In-normal statistics.
It is therefore a principal object of the invention to provide a system for simulating optical atmospheric effects in a laboratory.
It is a further object of the invention to provide an optical scintillator providing real-time statistical control over spatial intensity modulation of light sources used for laboratory analysis of optical receivers.
It is yet another object of the invention to provide a liquid mercury optical scintillator for simulating atmospheric effects in a laboratory.
These and other objects of the invention will become apparent as a derailed description of representative embodiments thereof proceeds.